Abstract

An intuitive model for the expansion of the universe is developed in which special relativity is used to describe events seen by a hypothetical observer in a Lorentz frame of reference. The cosmic microwave background photons he sees are the red-shifted remnants of hot photons emitted from the matter flying rapidly away from him. This special relativistic model, also called the Milne model, represents the extreme case of a Friedmann (general relativistic) universe in the limit of vanishingly small density of matter. The special relativistic model approximates an open universe (one that expands forever) with increasing accuracy as time evolves.